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The case study, believed to be the first of its kind, raises questions about whether allergens are transmitted by blood more often than anyone thinks. It's published in the latest issue of the New England Journal of Medicine.

“It is possible that allergens transferred in blood products to other patients have led to reactions that have gone unexplained and unreported,” wrote lead author Dr. Joannes F.M. Jacobs of Radboud University Nijmegen Medical Center in the Netherlands.

Currently, allergic reactions caused by blood or its components are believed to be very rare.

The boy, who had leukemia, was receiving a transfusion of pooled platelets at a Netherlands hospital last year when he developed a rash and skin swelling. His blood pressure dropped and he began having trouble breathing. It took an injection of adrenaline to resuscitate him, the report said.

Afterward, puzzled health care workers reviewed every possible cause for the reaction, including allergies to drugs or latex and common transfusion-related problems. Nothing clicked until they talked to the boy’s mom, who said her son had had a similar episode after eating peanuts at age 1. Since then, his parents had kept the nuts out of the child’s diet.

Doctors then tracked down the five blood donors who contributed platelets. Three of them said they’d eaten handfuls of peanuts the night before donating blood.

Peanut allergen resists digestion
It turns out that the major allergen in peanuts, Ara h2, is extremely resistant to digestion because of a peptide that can show up in blood serum for up to 24 hours after ingestion, researchers wrote. When doctors tested the boy’s blood, they found his levels of peanut-specific antibodies and the culprit peptide were far higher than normal.

"The patient proved to be allergic not merely to raw peanuts, but also for the protein that can end up in the blood," Jacobs wrote in an e-mail. "Such a case has never been described in literature before."

That indicates that the peanut-munching donors transmitted the allergen to the child, said the scientists.

The boy's existing allergy was sparked by the tainted blood, but other patients actually have developed new allergies after receiving blood from affected donors.

In 2007, for instance, an 80-year-old woman who'd recently received blood developed an anaphylactic reaction within minutes of eating peanut butter, according to a report in the journal Current Allergy and Asthma Reports. She’d never been allergic before, but two days earlier she’d been transfused with two units of blood from a 19-year-old donor who turned out to have a severe peanut allergy.

It’s not clear how often blood products trigger allergic reactions, and a leading U.S. blood expert says he can recall only sporadic reports.

“It’s got to be quite rare, given that we are transfusing 25 million blood components each year,” said Dr. James AuBuchon, president of the AABB, the nonprofit agency that promotes blood donation and transfusion in the U.S. “I’m grateful that this does not appear to be a large problem.”

Some critics have called for screening the diets of blood donors for potential triggers. However, that would be both impractical and costly for such a large volume of donors and recipients, said AuBuchon. More than 1 percent of the U.S. population, or 3 million people, are believed to be allergic to peanuts and tree nuts.